Patent GB-784678-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: CURTIS GERALD
IPC: H04Q3/62
Filed: 1956-05-18
Abstract: 784,678. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. May 18, 1956, No. 15505/56. Class 40 (4). In a PABX network, in which a parent exchange has a manual board by means of which calls incoming to the network are set up, a satellite exchange is connected to the parent exchange over a two-wire tie-line which is terminated at the satellite exchange by a relay set which by reversing potentials on the two-wire line indicates to the manual board operator, when she is setting up a call incoming to the network to a satellite subscriber, whether the wanted subscriber's line circuit is idle or busy. A single reversal of potential on the tie-line is given if the subscriber's line circuit is busy, or if all equipment in a particular stage is busy. If the subscriber's line circuit is idle and accessible, one reversal of a given duration is followed by another reversal of a given minimum duration. The parent exchange shown in the trunking diagram of Fig. 1 has an incoming main exchange line MEL terminated. in a circuit ELC which in conjunction with selectors ATF, BTF, provides an enquiry facility within the exchange as described in Specification 771,485; which Specification also describes incoming, outgoing, and local calls at the parent exchange. Call from a main exchange to a satellite exchange. Ringing current on an incoming line MEL is applied at circuit ELC to flash a manual board lamp CL to attract an operator who operates a key to connect with the calling line over a circuit CC and a line finder ELF. On application of a loop to the calling line ringing current is tripped and lamp SL lights enabling the operator to ascertain the wanted number. By putting a holding loop across the line MEL and marking a path NP, a group selector TGS is seized from which the connection can be made. The first digit sets TGS to a lever having access to twowire tie-lines over an outgoing auto-to-auto relay set OGAA marked idle by resistance battery R1 on the P lead of the multiple GSM, Fig. 2, while at the satellite exchange the twowire line terminates in an incoming auto-to-auto relay set ICAA, Fig. 3. When relay set OGAA is seized a loop is completed from the manual board and resistance battery is applied to the D lead of multiple GSM. In consequence relays A and B respond, relay B operating relay TA which locks up over its bottom winding and applies unbalancing earth on the two-wire line until relay TB, also operated by B but delayed, restores the balance. At the relay set ICAA in the satellite exchange the line extends over the differentially connected windings of relay ATA which operates when the line is unbalanced and causes ATB to respond and lock up when earth is returned over the P lead from group selector ICGS. ATB in locking up operates relay HF, to bridge the windings of ATA, and connects relays AES and ABS in series to the D lead of multiple ICGS. HF causes relay X to respond but without effect. Impulse trains,- corresponding to the second and subsequent digits transmitted from the manual board key sender, are now repeated by relay set OGAA and assuming the wanted subscriber's line circuit is idle and accessible over final selector SFS, full earth is returned over the D lead of multiple ICGS to operate relays AES, ABS. Relay AES operates relay Z and disconnects X which is slow to release. A contact of Z holds AES and ABS operated independently of the D lead while the remaining contacts of Z apply reversed battery to the tie-line and hold the sattelite final selector SFS with resistance R6. When X eventually relapses battery to the tie-line is again reversed and the slow release of Z is initiated. At relay set OGAA when the battery on the tie line is first reversed relay D responds over rectifier MR2 and operates relays DD and BS; relays AB, TA, and TB, being operated already. When battery on the tie-line is reversed a second time, D falls back, initiating the slow release of DD which when released energizes ES in series with BS; ES locking up to disable BS and connecting earth to the D lead of multiple GSM to indicate the idle condition to the manual board. Reply by the called satellite subscriber reverses potentials on the speech wires and replaces earth with battery on the D lead of multiple ICGS. This can have no effect before relay Z releases, when, or after which, ABS at ICAA falls back; AES being held over its upper winding, and HF being released to connect ATA into the line. Reversal of battery over the tieline operates D at OGAA, causing DD to respond and connect AS to operate in series with ES. AS reverses battery to the multiple GSM and replaces earth on the D lead by battery over relay CD which causes release of the manual board and connection of incoming line MEL to the path NP. When the manual board is released battery on the D lead from circuit CC is replaced by earth from circuit ELC to operate CD at OGAA which, with AS operated, connects earth to one side of the tie-line causing differential relay ATA to respond at ICAA at the satellite exchange. ATA operates AOP which locks up and releases AES and HF, also connecting transformer TR2 into the line. Relay AA responds over the loop circuit to complete the talking path and to energize relay AB which puts holding earth on the D lead of multiple ICGS to remove a speech battery bridge at the selector SFS. With the operation of AOP, battery is taken from the tie-line so that at OGAA relays D, DD and CD fall back, causing OP to respond and lock itself to the D lead of multiple GSM to disconnect transformer TR1 from the line and release relays AS and ES. The conversational path is now complete. An enquiry call can be made by a satellite subscriber connected to an exchange line MEL by pressing a button which unbalances his line in the multiple ICGS to operate differential relay DF to unbalance the tie-line. Circuit ELC responds to apply a holding loop across MEL and connect path NP to path EP over which other subscribers of the PABX network can be obtained and to which the line MEL may be transferred as described in Specification 771,485. Release of connection. When the satellite subscriber replaces his receiver AA falls back, initiating the slow release of AB and energizing ACD for as long as AB holds. AB. released takes earth from the D lead of multiple ICGS to release the satellite selectors which takes earth from the P lead and releases ATB and AOP to return relay set ICAA to normal. When the tie-line loop is broken at ICAA, circuit ELC in the parent exchange releases and takes earth from the D lead of multiple GSM, releasing OP in the relay set OGAA. This short-circuits relay B which when released initiates the slow release of TB and releases TA and CD which pulled back as soon as OP released. TB is delayed to cover release of the relay set ICAA and when released connects idle marking battery to the P lead. Busy condition returned by satellite. Trunk offering. If the subscriber's line circuit is busy, or if no selector is available in a particular stage, high resistance earth is returned on the D lead of multiple ICGS and is sufficient to operate relay ABS, but not relay AES, in relay set ICAA. Reversal of battery on the tie line caused by ABS operating operates relay D in relay set OGAA where relays A, B, TA, and TB are already made and relays DD and BS are operated in consequence; BS connecting earthed resistance R8 to the D lead which causes a lamp to flash on and off at the manual board to signal the busy condition. To offer the incoming trunk the operator dials " O," dialling relay CD at OGAA holding up during the impulse train which is just sufficiently long for a contact of CD to charge capacitor C5 to a point where relay PF can respond and lock up in dependence on DD and BS. PF made releases BS, as CD falls back at the end of the impulse train initiating the slow release of DD. BS on falling back disconnects R8 from the D lead to cancel the supervisory signal. When DD finally relapses PF is released. At the satellite exchange the final selector SFS responds to the digit " O " to break in to the busy subscriber's connection and takes high resistance earth from the D lead of multiple ICGS, releasing ABS at ICAA to restore the tie-line potentials and release D at OGAA. If the subscriber clears his connection SFS responds to connect with his now idle line circuit. If the busy signal is due to the line circuit not being accessible, trunk offering is abortive and the condition becomes apparent. Call from parent exchange to satellite. A subscriber to the parent exchange seizes a selector GS and the first digit seizes an idle tie-line over a relay set OGAA in which relays A, B, and TB, respond when a loop is made. As the D lead of multiple GSM is not marked by battery relay TB does not respond. Relay A extends the loop to seize a selector SGS at the satellite which returns earth on the P lead of multiple ICGS to operate relay HF bridging differential relay ATA and providing a direct path over which further digits may be dialled to set up the call and over which conversation may be conducted. Reply of the called subscriber reverses the line potentials to operate relays D, DD, and AS at OGAA; AS reversing the line potentials without effect as no supervision is exercised. When the subscriber to the parent exchange clears, relay A at OGAA releases, disconnecting D and short-circuiting B. When D falls back DD is released and when B relapses TB is disconnected to release when capacitor C1 discharges, during which time the satellite selector stages release in response to the broken loop and relinquish HF at ICAA. When TB falls back OGAA is again marked idle.